The present invention relates generally to a method and apparatus for controlling the wall profile of laser machined features, such as holes and grooves. More specifically, it relates to a system for moving a laser beam relative to a workpiece to machine the walls of the machined features therein to achieve a desired profile.
In the manufacture of certain products, features, such as holes and grooves, are produced in workpieces by ablating the workpiece material with a laser beam, e.g., Gugger U.S. Pat. No. 3,576,965. When a laser, such as an excimer laser, ablates a hole or groove, it exhibits a small xe2x80x9cnaturalxe2x80x9d taper. A taper of 5-9 degrees half angle is typical for laser machined holes in many plastics such as polyamide and polyester. This natural taper varies slowly with laser beam energy density, wavelength and numerical aperture. However, these parameters are difficult to control. Moreover, where a larger taper angle is required, it is necessary to use a different approach.
One approach involves controlling (a) the angle of the laser beam striking the workpiece relative to the axis of a hole being machined and (b) the distance between the intersection of the beam with the surface of the workpiece and the axis of the hole. Corfe et al. U.S. Pat. No. 5,043,553. This approach is, however, difficult to implement because of its inherent complexity and, for an excimer laser, because of the difficulty in focusing the laser across the image field.
Other approaches attempt to control wall taper angle by controlling the fluence used in a laser beam or by refocusing a projection lens. These approaches have only limited success. See Sheets U.S. Pat. No. 4,940,881.
Finally, another approach involves displacing a laser beam relative to a workpiece. One way to do this involves interposing a thin refractive disc in the laser beam and rotating the refractive disc not about its axis but instead about the optical axis or an axis parallel to it, a so-called xe2x80x9cwobble plate.xe2x80x9d In a preferred embodiment, the refractive disc has parallel faces and is inclined at a small angle to a reference plane perpendicular to the optical axis. In another embodiment, the faces of the refractive element are at a small angle to each other. Sheets U.S. Pat. No. 4,940,881. In the preferred embodiment, the taper angle of the wall is dependent on the angle at which the refractive element is inclined. It has, however, proven time consuming to change that angle in practice, and the required apparatus for rotating the refractive disc is relatively large and complicated.
It is an object of the present invention to move a laser beam relative to a workpiece to machine the taper angle of the walls of laser machined features therein, such as holes and grooves,at specified angles, which angles may be readily and remotely changed. It is also an object of the present invention to move a laser beam relative to a workpiece to machine the walls of the machined features therein to achieve other desired profiles.
A new method and apparatus for moving an excimer laser beam relative to a workpiece to control the wall profile of laser machined features, such as holes and grooves. In a preferred embodiment, an excimer laser beam is displaced relative to a workpiece in a substantially circular motion and the substantially circular motion is further displaced relative to the workpiece to correspond to a desired shape.
The foregoing and other objects, features and advantages of the current invention will be apparent from the following detailed description of preferred embodiments of the invention as illustrated in the accompanying drawings.